1. Technical Field
The invention relates to devices that generate undesirable acoustic noise, such as electrical or electro-mechanical assemblies. More particularly, the invention relates to the mitigation of noise generated by such assemblies in personal computers.
2. Description of the Prior Art
FIG. 1 is an exploded view of a typical personal computer having a tower configuration. The personal computer comprises a power supply 1 shown fitted to a case 2, a front cover 3, a floppy and hard drive controller 4, a video card 5, a floppy drive 6, a hard drive 7, a main or motherboard 8, an Input/Output (I/O) card 9, and a 5.25-inch floppy drive 10. Other boards or components may be added as required such as sound cards, CD ROM drives, or network adapters (not shown).
FIG. 2 shows the noise emissions at 1 meter for a typical desk top personal computer compared to the maximum permissible sound pressure levels in octave bands for audiometric conditions (audibility), when the listener's ears are not covered. The histogram shows that at all frequencies from 250 Hz to 4 kHz, the personal computer exceeds the permitted level by as much as 15 dB. This constant noise contributes to fatigue and irritation and, in fields such as sound recording or hearing evaluation, also precludes the presence of a computer in the recording room or hearing evaluation facility.
FIG. 3 is an exploded view of a typical hard drive for a personal computer. A metal frame 12 supports a motor 13 which drives the data storage disk 14. A servo mechanism 15 moves a read/write head 16 across the data storage disk 14. The head 16 is extremely close to, but not in contact with, the surface of the data storage disk 14. Variations in current flowing in coils located in the head 16 enable data to be written to the data storage disk 14 in the form of zeros or ones. Circuit boards 17a and 17b (see FIG. 4) containing various electrical components that comprise the driver hardware for the servo mechanism are coupled by a plug 18 to the hard drive controller 4 (see FIG. 1). A metal cover 19 prevents damage to the drive.
The motor 13 and the servo mechanism 15 in the disk drive are both sources of noise. In most computers, the hard drive motor 13 runs continuously. However, in laptop computers and notebook computers, sophisticated power management circuitry may turn the hard drive off when it is not needed. The noise from the servo mechanism is intermittent, occurring when data are being read or written.
Located within the computer tower is a power supply 1 (see FIG. 1). A typical desktop computer requires 100 to 250 Watts of power, some of which is converted to heat during operation of the computer. Natural convection and radiant heat loss from the computer case 2 is typically insufficient to conduct and dissipate the generated heat from the internal components of the personal computer. To prevent heat build-up within the personal computer, one or more cooling fans, vented to the external atmosphere are used to provide a flow of cooling air. Such cooling fans are also a source of noise within the personal computer.
Floppy disk and CD ROM drives are other sources of acoustic noise. They differ from the cooling fans and the hard drive only in that they are not usually continuous sources of noise. Floppy disks are typically used to transfer data from one computer to another and, because of that, are used infrequently and do not produce continual background noise, which is annoying to the user or which may interfere with various activities, such as recording or hearing evaluation. Similarly CD-ROM drives are used intermittently.
It is known to reduce noise generated by electro-mechanical devices, such as by various assemblies within personal computers, by surrounding the source of such noise within the device with a sound barrier material. However, such treatment reduces or eliminates convection cooling within the device, especially with regard to any assemblies that have been surrounded by such sound absorbing material. This lack of convective cooling in connection with such assemblies can lead to their overheating and failure.
The use of acoustic shielding to reduce noise levels is well known to those skilled in the arts. See, for example G. Wyler, Silent Disk Drive Assembly, U.S. Pat. No. 5,510,954 (Apr. 23, 1996), which discloses an acoustically shielded hard disk drive that uses a sound barrier layer to surround the disk drive assembly, and that provides a heat conductive path through the sound barrier layer to an externally mounted heat sink assembly.
It is commonly known to use heat sinks to improve the convective cooling of electronic components. It is also known to remove heat from the outer surfaces of a packaged device, such as a hard disk drive by the use of such techniques as surrounding the drive with a liquid heat sink in a pouch or by conducting heat to an external heat sink (see, for example Wyler, supra).
Unfortunately, the prior art does not address the problem of local overheating of components within a particular electro-mechanical assembly, such as a disk drive circuit board, that results when the convective air flow around the assembly is effectively eliminated by the placement of acoustic shielding around the assembly. Nor does the prior art provide a technique for accomplishing such acoustic shielding without the use of a bulky external heat sink that adds to the cost and complexity of the device (see, for example Wyler, supra).